The possiblility that Jon may have had vocal cord surgery somewhere between 2015 and the 2016 THINFS promo shows has been explored a few times on this board.
People have pointed to the fact that Jon noticably can't perform basic songs like Lost Highway,Born To Follow and Leavin' Goin On properly like he did years prior,when he performs those songs his tone is completely off and he's usually off tempo he kinda sounds breathy. His lower vocal register sounds fine it's actually deepened quite a bit since 2013,his mid range is somewhat shot,as for his high register it is also shot. Jon's mid register sounds shot because he seems to lack breathing support,the same goes for his high register.

When you look at Jon's face when he performs songs like Born To Be Your Baby,Bad Name,It's My Life and Bad Medicine, he looks like he's going to pass out which means he is lacking breathing support to sustain the mids and highs in each song.

There are two obvious scenarios that might have caused Jon's lack of breathing support,
1. Smoking (Not sure if he smokes these days but I heard he quit in 2010)
2. Jon had vocal cord surgery in an attempt to fix whatever was happening with his voice and it backfired on him and he lost any range he had left and affected his breathing when singing.

There are obvious signs to support the latter,also in 2014 right up until the 2016 promo shows Jon sounded like Jon,he still had that distinct tone in his voice that even with your eyes closed you'd know it was JBJ singing. These days Jon sounds unrecognisable even his low registers sounds way different. Even a change in vocal technique isn't enough to completely change the tone in a singer's voice.